Hi all,
I'm an owner of a duplex and have a service entrance of 200 amp. It is split into two to service my tenant's and my floor. The upper floor has a 100 amp main breaker, but I have a 150 amp main breaker.
My question is, what amperage do I really have to work with? 150 or 100?

Where thee service entrance splits is there breakers for each apartment? If there is then those breakers determine your service size. If it is a main lug and there are no breakers then you have a 150 amp service.

You would have to do a load calculation to know for sure, but ya, you have 150As. Lots of multi unit buildings have main breakers that are less then the sum of the individual unit disconnects. The sizing of the main is based on a load calculation (found in the NEC) which adds up the expected use of each unit and comes up with an expected load. The load calculations are based on the assumption that not all the units are going to draw max power at the same time. Sometimes one tenant will be drawing alot and sometimes another and rarely both at the same time.

Thanks guys.
So yea, I checked the distribution panel where the main is split and no breakers there. So 150A it is.

I got a quote for an oil to electric furnace conversion where they said I couldn't install the electric furnace because I only had 100A and that I would have to do a service upgrade. I'm now thinking that I don't need the upgrade since 150 is enough for my needs.

Here are my main appliances: electric oven, dishwasher, washer, dryer, fridge, water heater. There used to be second oven in the basement kitchen but I don't use it and will eventually remove that wiring/breaker. This will free up an additional 40A space.

Take a picture of the panel. The main will tell you what the panel has. Now if you get a new furnace that is for both units I assume not just yours. That would mean you would have a zoned system and only one zone can call for heat at a time. But if you are installing two then you will most likely not have enough service.

Now from what I know it you have 150 amp service you can upgrade to a 200 amp by swapping out the panel and double checking the wire size from the socket to the main.

I have done a main panel upgrade and there is not much to it. With some basic knowledge and planning things went great costing me less then $300 to go from a 150 amp federal pacific (we all know the issue with those) to a 200 amp square D QO. Now that is also going with the fact that in MN as the home owner I can do all of my own work with permits.

I have done a main panel upgrade and there is not much to it. With some basic knowledge and planning things went great costing me less then $300 to go from a 150 amp federal pacific (we all know the issue with those) to a 200 amp square D QO. Now that is also going with the fact that in MN as the home owner I can do all of my own work with permits.

You are forgetting the fact that the OP has a double service that is already 200A. He CANNOT upgrade his side to 200A without upgrading the whole service to 320/400. Not exactly DIY IMO, regardless of how easy your was.